1. Field of the Invention
This invention relates generally to the field of X-ray imaging detectors and more particularly to a system and method for controlling frame scanning to avoid artifacts created by X-ray pulses during the imaging process.
2. Description of the Prior Art
Photosensitive element arrays for converting incident radiant energy into an electrical signal are commonly used in imaging applications, for example, in x-ray imagers and facsimile device arrays. Real time and continuous image acquisitions of pulsed X-ray imaging systems are required in many applications. These applications include high energy X-ray (therapy), medium energy (industrial), or low X-ray energy (diagnostics). For Megavoltage (MV) imaging the pulsing period is normally small compared to detector frame scan time and pulsing artifacts of the linear accelerator are often present in current imaging systems. For pulsed KV X-ray the period of exposure is normally longer than frame scan time, however, there is often a significant uttering in KV beam pulses that makes the offset calibration very difficult and inaccurate if a frame synchronization scheme is used after radiation delivery. This makes the frame overhead time longer if the radiation delivered during the frame delay time.
Cone beam (CB) or even fan beam Computed Tomography (CT) image acquisition also often uses pulsed X-ray generation. The current technique mainly used for CBCT with pulsed X-ray is based on frame synchronization, i.e. radiation delivered for each specific angle and the frame readout occurs after radiation delivery. In this mode the detector is controlled externally and frame scan is synchronized with the radiation on or off signal of X-ray source. There are some problems with this technique. Frame synchronization does not work when the beam pulsing frequency is greater than the frame scan rate (or in other words, the timing period between two pulses is smaller than frame scan time) and when pulsing frequency varies during radiation. Frame integration time is equal to the sum of beam on time, frame scan time, and wait time until next radiation pulse occurs. Therefore, the frame integration time will be longer and dark current shot noise will be greater for longer frame integration time. Due to the jittering of the delivered radiation pulses which sometimes is not avoidable (for instance if the gantry angle controls the shooting of the X-ray exposure), the dark offset correction will not be optimum and this will cause artifacts in the reconstructed CT images and limit low dose applications.
It is therefore desirable to provide a system and method for frame scanning control for flat panel imagers that are optimized for pulsed x-ray imaging by capitalizing on the adaptive nature of frame scan. It is also desirable that the system provide a control scheme for detector line scan with dependency on the X-ray control signals.